.\" -*- nroff -*-
.\" Copyright 2010 Cisco Systems, Inc.  All rights reserved.
.\" Copyright 2006-2008 Sun Microsystems, Inc.
.\" Copyright (c) 1996 Thinking Machines Corporation
.\" Copyright (c) 2020      Google, LLC. All rights reserved.
.\" Copyright (c) 2020      FUJITSU LIMITED.  All rights reserved.
.\" $COPYRIGHT$
.TH MPI_Request_free 3 "#OMPI_DATE#" "#PACKAGE_VERSION#" "#PACKAGE_NAME#"
.SH NAME
\fBMPI_Request_free\fP \- Frees a communication request object.

.SH SYNTAX
.ft R
.SH C Syntax
.nf
#include <mpi.h>
int MPI_Request_free(MPI_Request *request)

.fi
.SH Fortran Syntax
.nf
USE MPI
! or the older form: INCLUDE 'mpif.h'
MPI_REQUEST_FREE(REQUEST, IERROR)
	INTEGER	REQUEST, IERROR

.fi
.SH Fortran 2008 Syntax
.nf
USE mpi_f08
MPI_Request_free(\fIrequest\fP, \fIierror\fP)
	TYPE(MPI_Request), INTENT(INOUT) :: \fIrequest\fP
	INTEGER, OPTIONAL, INTENT(OUT) :: \fIierror\fP

.fi
.SH INPUT/OUTPUT PARAMETER
.ft R
.TP 1i
request
      Communication request (handle).

.SH DESCRIPTION
.ft R
This operation allows a request object to be deallocated without waiting for the associated communication to complete.
.sp
MPI_Request_free marks the request object for deallocation and sets request
to MPI_REQUEST_NULL. Any ongoing communication that is associated with the request will be allowed to complete. The request will be deallocated only after its completion.

.SH NOTES
Once a request is freed by a call to MPI_Request_free, it is not possible to check for the successful completion of the associated communication with calls to MPI_Wait or MPI_Test. Also, if an error occurs subsequently during the communication, an error code cannot be returned to the user -- such an error must be treated as fatal. Questions arise as to how one knows when the operations have completed when using MPI_Request_free. Depending on the program logic, there may be other ways in which the program knows that certain operations have completed and this makes usage of MPI_Request_free practical. For example, an active send request could be freed when the logic of the program is such that the receiver sends a reply to the message sent -- the arrival of the reply informs the sender that the send has completed and the send buffer can be reused. An active receive request should never be freed, as the receiver will have no way to verify that the receive has completed and the receive buffer can be reused.

.sp
\fBExample:\fR
.sp
.nf
    CALL MPI_COMM_RANK(MPI_COMM_WORLD, rank)
    IF(rank.EQ.0) THEN
        DO i=1, n
          CALL MPI_ISEND(outval, 1, MPI_REAL, 1, 0, req, ierr)
          CALL MPI_REQUEST_FREE(req, ierr)
          CALL MPI_IRECV(inval, 1, MPI_REAL, 1, 0, req, ierr)
          CALL MPI_WAIT(req, status, ierr)
        END DO
    ELSE    ! rank.EQ.1
        CALL MPI_IRECV(inval, 1, MPI_REAL, 0, 0, req, ierr)
        CALL MPI_WAIT(req, status)
        DO I=1, n-1
           CALL MPI_ISEND(outval, 1, MPI_REAL, 0, 0, req, ierr)
           CALL MPI_REQUEST_FREE(req, ierr)
           CALL MPI_IRECV(inval, 1, MPI_REAL, 0, 0, req, ierr)
           CALL MPI_WAIT(req, status, ierr)
        END DO
        CALL MPI_ISEND(outval, 1, MPI_REAL, 0, 0, req, ierr)
        CALL MPI_WAIT(req, status)
    END IF
.fi
.sp
This routine is normally used to free persistent requests created with
either
.I MPI_Recv_init
or
.I MPI_Send_init
and friends.  However, it can be
used to free a request created with
.I MPI_Irecv
or
.I MPI_Isend
and friends;
in that case the use can not use the test/wait routines on the request.

It
.B is
permitted to free an active request.  However, once freed, you can not
use the request in a wait or test routine (e.g.,
.I MPI_Wait
).

.SH ERRORS
Almost all MPI routines return an error value; C routines as the value of the function and Fortran routines in the last argument.
.sp
Before the error value is returned, the current MPI error handler is
called. By default, this error handler aborts the MPI job, except for I/O function errors. The error handler may be changed with MPI_Comm_set_errhandler; the predefined error handler MPI_ERRORS_RETURN may be used to cause error values to be returned. Note that MPI does not guarantee that an MPI program can continue past an error.

.SH SEE ALSO
MPI_Isend
.br
MPI_Irecv
.br
MPI_Issend
.br
MPI_Ibsend
.br
MPI_Irsend
.br
MPI_Recv_init
.br
MPI_Send_init
.br
MPI_Ssend_init
.br
MPI_Rsend_init
.br
MPI_Test
.br
MPI_Wait
.br
MPI_Waitall
.br
MPI_Waitany
.br
MPI_Waitsome
.br
MPI_Testall
.br
MPI_Testany
.br
MPI_Testsome



